Known piston pumps each used for a hydraulic brake apparatus for a vehicle are disclosed in JP2001508854A (Reference 1), DE102005034571A1 (Reference 2), and DE10053992A1 (reference 3).
According to the piston pump disclosed in the Reference 1, an annular-shaped body is attached onto an outer periphery of a piston that is driven by a cam so as to perform a reciprocating movement. The body is axially displaceable (i.e., slidable) on the piston and is pressed towards one side by a spring. The body and the piston are inserted into a hydraulic block (precisely, a cylinder formed at the hydraulic block) while respective end portions of the body and the piston face a same pump chamber (i.e., positive displacement chamber).
According to the piston pump disclosed, the piston constitutes a first pump while the body constitutes a second pump. In cases where a thrust force caused by a pressure within the pump chamber is smaller than a pre-stressing force of the spring, the body moves together with the piston. On the other hand, in cases where the thrust force caused by the pressure within the pump chamber is larger than the pre-stressing force of the spring, the body is pressed towards a movement end portion thereof that is located in a direction away from the pump chamber.
That is, until the pressure in the pump chamber reaches a predetermined value (i.e., pump discharge pressure is low), the first pump and the second pump are operated together. When the pressure in the pump chamber exceeds the predetermined value (i.e., pump discharge pressure is high), the operation of the second pump is stopped and only the first pump is operated. Accordingly, a pump discharge volume is changed between cases where the pump discharge pressure is high and low.
In the same way as the piston pump in the Reference 1, according to the piston pumps disclosed in the References 2 and 3, respectively, the operation of the second pump is stopped when the pressure in the pump chamber exceeds a predetermined value though structures of the piston pumps in the References 1, 2, and 3 are slightly different from one another.
The piston pump disclosed in each of the References 1, 2, and 3 can increase the pump discharge volume at a time of low pump discharge pressure as compared to the pump discharge volume at a time of high discharge pressure without an increase of capacity of a motor that drives the piston pump (i.e., piston driving motor). Accordingly, in the ABS control, for example, a fluid stored in a reservoir (i.e., brake fluid) can be rapidly pumped. In addition, in the ESC control, the TRC control, and the like, a responsiveness of pressurization of the wheel cylinder pressure can be enhanced to thereby improve each control performance.
As mentioned above, the piston pump disclosed in each of the References 1, 2, and 3 can increase the pump discharge volume at a time of low pump discharge pressure as compared to the pump discharge volume at a time of high discharge pressure without an increase of capacity of the piston driving motor to thereby enhance responsiveness of the ABS control, the ESC control, and the like. However, the pressure in the pump chamber upon the change of the pump discharge volume may not be stable.
That is, the pressure in the pump chamber when the pump discharge volume is changed (i.e., the operation of the second pump is stopped) is determined on the basis of a sum of a load of the spring that biases the body and a sliding resistance of the body. In this case, the sliding resistance of the body is variable depending on a size of the sliding portion, temperature, and the like. Accordingly, the pressure in the pump chamber when the pump discharge volume is changed varies due to the variation in the sliding resistance of the body, which may negatively affect the stability of the hydraulic control.
A need thus exists for a piston pump which is not susceptible to the drawback mentioned above.